Optical system having an improved signal-to-noise ratio of eye-tracking

ABSTRACT

An optical system includes a grating including at least one substrate and a grating structure coupled to the at least one substrate. The grating structure is configured to diffract a first light having an incidence angle within a predetermined range. The optical system also includes a polarizer configured to transmit the first light diffracted by the grating structure and block a second light reflected by a surface of the at least one substrate.

BACKGROUND

Head-Mounted Display (HMD) has been widely used in, e.g., videoplayback, gaming, and sports. HMDs have been used to realize virtualreality (VR), augmented reality (AR) or mixed reality (MR). Some VR, ARor MR applications require an eye tracking function that monitors theeye of the user and/or the region surrounding the eye of the user. Bymonitoring the eye and/or the surrounding region, the HMD can determinea gaze direction of the user, which can be used for improving displayquality, performance, and/or user experience, and can be used to addressvergence/accommodation conflict. Further, by monitoring the eye and/orthe surrounding region, the HMD can estimate the psychological stateand/or changes in the psychological state of the user, as well asphysical characteristics of the user. The above information can be usedby the HMD to, e.g., determine what content to provide to the user. Forexample, if the user is concentrating on a particular task or activity,the HMD may determine that the user prefers not to be interrupted withother information unless such information would be important to theuser. Various eye-tracking techniques have been used in HMDs. However,surface reflection at various optical elements used for eye-trackingoften introduces noise to an eye-tracking signal, reducing thesignal-to-noise ratio of the eye-tracking signal and degrading theaccuracy of the eye-tracking.

BRIEF SUMMARY OF THE DISCLOSURE

One aspect of the present disclosure provides an optical device. Theoptical device includes an optical system. The optical system includes agrating including at least one substrate and a grating structure coupledto the at least one substrate. The grating structure is configured todiffract a first light having an incidence angle within a predeterminedrange. The optical system also includes a polarizer configured totransmit the first light diffracted by the grating structure and block asecond light reflected by a surface of the at least one substrate

Another aspect of the present disclosure provides an eye-trackingsystem. The eye-tracking system includes a light source configured toemit a light to illuminate an eye of a user. The eye-tracking systemalso includes a grating disposed facing the eye and configured toreceive the light reflected by the eye. The grating includes at leastone substrate and a grating structure coupled to the at least onesubstrate. The grating structure is configured to diffract a first lightof the light having an incidence angle within a predetermined range. Theeye-tracking system also includes a polarizer disposed facing thegrating and configured to transmit the first light diffracted by thegrating structure and block a second light of the light reflected by asurface of the at least one substrate. The eye-tracking system furtherincludes an optical sensor disposed downstream of the polarizer inoptical series and configured to receive the first light transmittedthrough the polarizer and generate an image of the eye based on thereceived first light.

Other aspects of the present disclosure can be understood by thoseskilled in the art in light of the description, the claims, and thedrawings of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The following drawings are provided for illustrative purposes accordingto various disclosed embodiments and are not intended to limit the scopeof the present disclosure.

FIG. 1 illustrates an optical system for eye-tracking in a head-mounteddisplay (“HMD”);

FIG. 2 illustrates an eye-tracking system to demonstrate an effect ofsurface reflection;

FIG. 3 shows a plot of Fresnel transmittance and reflectance versusincidence angle at an air-glass interface;

FIG. 4 illustrates an eye-tracking system having a surface reflectionnoise reduction mechanism, according to an embodiment of the disclosure;

FIG. 5A illustrates an experimental result of the eye-tracking systemshown in FIG. 2;

FIG. 5B illustrates an experimental result of the eye-tracking systemshown in FIG. 4, according to an embodiment of the disclosure;

FIG. 6 illustrates a schematic diagram of another eye-tracking systemhaving a surface reflection noise reduction mechanism, according to anembodiment of the disclosure;

FIG. 7A illustrates an experimental result of an eye-tracking without asurface reflection noise reduction mechanism;

FIG. 7B illustrates an experimental result for the eye-tracking systemin FIG. 6 having a surface reflection noise reduction mechanism,according to an embodiment of the disclosure;

FIG. 8A is a cross-sectional view of a polarization volume hologram(“PVH”) layer included in a PVH grating, according to an embodiment ofthe disclosure;

FIG. 8B is a partial plan view of the PVH layer shown in FIG. 8A in thex-y plane, according to an embodiment of the disclosure;

FIG. 8C is another partial plan view of the PVH layer shown in FIG. 8Ain the x-y plane, according to an embodiment of the disclosure;

FIG. 9A is a schematic diagram of an HMD, according to an embodiment ofthe disclosure; and

FIG. 9B is a cross sectional view of a front rigid body of the HMD shownin FIG. 9A, according to an embodiment of the disclosure.

DETAILED DESCRIPTION

Reference will now be made in detail to exemplary embodiments of thedisclosure, which are illustrated in the accompanying drawings.Hereinafter, embodiments consistent with the disclosure will bedescribed with reference to drawings. In the drawings, the shape andsize may be exaggerated, distorted, or simplified for clarity. Whereverpossible, the same reference numbers will be used throughout thedrawings to refer to the same or like parts, and a detailed descriptionthereof may be omitted.

Further, in the present disclosure, the disclosed embodiments and thefeatures of the disclosed embodiments may be combined under conditionswithout conflicts. It is apparent that the described embodiments aresome but not all of the embodiments of the present disclosure. Based onthe disclosed embodiments, persons of ordinary skill in the art mayderive other embodiments consistent with the present disclosure, all ofwhich are within the scope of the present disclosure.

Various eye-tracking techniques have been used in HMDs. For example, aninfrared light source may emit an infrared light to illuminate one ortwo eyes of a user of the HMD. The infrared light is not visible to thehuman eye and thus, does not distract the user wearing the HMD duringoperation. An optical sensor, such as a camera, may be arranged toreceive the infrared light that is reflected by the eye and generate animage of the eye based on the received infrared light. The image of theeye may be used to extract desired information (e.g., gaze direction,movement direction, psychological state, etc.) of the eye. Inconventional technologies, an eye-tracking optical element such as agrating may be arranged facing the eye and the optical sensor, infraredlight reflected by the eye may be diffracted by the grating towards theoptical sensor.

However, when the infrared light reflected by the eye is incident ontothe grating at an oblique angle, surface reflection may occur at asubstrate surface of the grating. Some of the surface reflection may bereceived by the optical sensor (e.g., camera) and some may be out of thereceiving area or coverage area of the optical sensor. The surfacereflection received by the optical sensor may introduce noise to aneye-tracking signal generated based on the infrared light propagating ina signal path, where the infrared light reflected by the eyes issubstantially normally incident onto the grating with zero or negligiblesurface reflection. For example, the optical sensor may generate abright main image of the eye superimposed with a darkened ghost image.That is, the signal-to-noise ratio of an eye-tracking signal may bereduced. The surface reflection noise light, if not processed, mayreduce the quality of the main image of the eye. As a result, theeye-tracking accuracy may be degraded.

In view of the surface reflection, the present disclosure provides anoptical system with an improved signal-to-noise ratio of a targettracking. The optical system may be used for eye-tracking in HMDs forAR, VR or MR applications. The optical system may include a surfacereflection noise reduction mechanism, which is configured tosignificantly reduce or eliminate surface reflection noise light in thelight received by an optical sensor. For example, when the opticalsystem is used for eye-tracking, the optical system may reduce noisesignals in a captured image of the eye, thereby improving thesignal-to-noise ratio and enhancing the accuracy of the eye-tracking.

In some embodiments, the optical system may include at least onesubstrate, a grating structure coupled to the at least one substrate, anoptical sensor, and a polarizer disposed in front of the optical sensorin optical series. The grating structure may be configured to guide alight reflected by an eye of a user towards the optical sensor. In someembodiments, the optical system may further include a light sourceconfigured to illuminate the eye of the user. Light reflected at asurface of the at least one substrate (referred to as surfacereflection) and guided by the grating structure towards the opticalsensor is a noise light. Light substantially normally incident onto thegrating structure and diffracted by the grating structure towards theoptical sensor is a signal light. The signal light and the noise lightmay be configured to have different polarization. The polarizer disposedin front of the optical sensor may be configured to block the noiselight but transmit the signal light. For example, the noise light may beconfigured to be substantially s-polarized, the signal light may beconfigured to be p-polarized or unpolarized, and the polarizer may beconfigured to block the s-polarized light and transmit the p-polarizedlight or the p-polarized portion of the unpolarized light. Thus, theoptical sensor may receive only the signal light that is transmittedthrough the polarizer, and generate an image of the eye based on thesignal light. The disclosed optical system may improve thesignal-to-noise ratio of the eye-tracking and, accordingly, enhance theaccuracy and quality of the eye-tracking.

FIG. 1 illustrates an optical system 100 implemented in an HMD. Theoptical system 100 may generate images by utilizing light emitted orreflected by a target being tracked, such as the user's eye. Fordiscussion purpose, such an optical system 100 is referred to as aneye-tracking system in the following descriptions. It is understood thatthe optical system 100 may be used to track a target other than an eyeof a user. In some embodiments, the eye-tracking system 100 may includea light source 105 configured to emit a light to illuminate one or twoeyes 115 of a user. The light source 105 may be positioned out of a lineof sight of the user and below the eye 115. FIG. 1 shows one eye 115 forillustrative purposes. It is understood that components for tracking theeye 115 may be replicated for tracking the other eye of the user, whichare omitted in FIG. 1.

In some embodiments, the light emitted by the light source 105 mayinclude a narrow spectrum or a relatively broad spectrum, and one ormore wavelengths of the light may be in the infrared (“IR”) spectrum,i.e., the spectrum of the light source 105 may be within, overlap, orencompass the IR spectrum. In some embodiments, the light source 105 mayemit light in the near infrared (“NIR”) band (about 750 nm to 1250 nm),or some other portion of the electromagnetic spectrum. NIR spectrumlight may be advantageous in some embodiments because the NIR spectrumlight is not visible to the human eye and thus, does not distract theuser wearing the NED during operation. The infrared light may bereflected by a pupil area, of the eye 115, the entire eye 115 of theuser, an area near, such as above, below, left to, or right to, the eye115 of the user, or an area including the eye 115 and the area near theeye 115.

The eye-tracking system 100 may include a grating 120 configured toguide the light reflected by the eye 115 towards an optical sensor 110.The optical sensor 110 may be arranged facing the grating 120, andconfigured to receive the light guided by the grating 120 and generate asignal for eye-tracking, such as an image of the eye 115 based on thereceived light. The optical sensor 110 may be sensible to light having awavelength within a spectrum that includes IR spectrum. In someembodiments, the optical sensor 110 may be sensible to IR light but notvisible light. In some embodiments, the optical sensor 110 may include acamera, such as a charge-coupled device (“CCD”) camera, a complementarymetal-oxide-semiconductor (“CMOS”) sensor, an N-typemetal-oxide-semiconductor (“NMOS”) sensor, a pixelated camera, or anyother suitable cameras.

Further, the optical sensor 110 may include a processor configured toprocess the infrared light, to generate an image of the eye 115, and/orto analyze the image of the eye 115 to obtain information that may beused for eye-tracking and other subsequent operations, such as fordetermining what information to present to the user or the layout of thepresentation of the information, etc. In some embodiments, the opticalsensor 110 may also include a non-transitory computer-readable storagemedium (e.g., a computer-readable memory) configured to store data, suchas the generated images. In some embodiments, the non-transitorycomputer-readable storage medium may store codes or instructions thatmay be executable by the processor to perform various steps of anymethod disclosed herein. In some embodiments, the processor and thenon-transitory computer-readable medium may be provided separately fromthe optical sensor 110. For example, the eye-tracking system 100 mayinclude a controller communicatively connected with the optical sensor110 and configured to receive data from the optical sensor 110. Thecontroller may be configured to analyze the data (e.g., images of theeye 115) received from the optical sensor 110 to obtain information foreye-tracking or other purposes.

In some embodiments, the grating 120 may include at least one substrate125 and a layer of grating structures 130 formed on the at least onesubstrate 125. The substrate 125 may provide support and protection tothe grating structures 130. In some embodiments, the substrate 125 mayinclude a lens or an optical waveguide made of a suitable material, suchas glass, plastics, etc. The substrate 125 may be rigid or flexible. Insome embodiments, the substrate 125 may also be a part of anotheroptical device or another optoelectrical device. For example, thesubstrate 125 may be a part of a functional device, such as a displayscreen. In some embodiments, the grating structures 130 may be formed onor bonded to a surface of the substrate 125. In some embodiments, thegrating structures 130 may direct contact the surface of the at leastone substrate 125. In some embodiments, the grating structures 130 maybe spaced apart from the substrate 125 by a spacing material. In someembodiments, additional layer(s), such as protection layer(s) and/orbuffer layer(s), can be arranged between the substrate 125 and thegrating structures 130.

The grating structures 130 may include any suitable grating structure.In some embodiments, the grating 120 may include a holographic opticalelement (“HOE”). In some embodiments, the grating 120 may include apolarization selective/sensitive grating, such as a polarization volumehologram (“PVH”) grating. In some embodiments, the grating 120 mayinclude a non-polarization selective/sensitive grating, such as a volumeBragg grating (“VBG”). In some embodiments, the grating structures 130and the substrate 125 may be made of the same material, and the gratingstructures 130 and the substrate 125 may be a single component, ratherthan being separate components. For example, the grating 120 may be aVBG where the substrate 125 is a glass block and the grating structures130 are integrally formed within the glass block. In some embodiments,the grating structures 130 and the substrate 125 may be made ofdifferent materials. For example, the grating 120 may be a PVH gratingwhere the grating structures 130 is formed by liquid crystals (“LCs”)and the substrate 125 is made of glass, i.e., the PVH grating includesan LC layer having a grating function formed on the substrate.

As shown in FIG. 1, the grating 120 may be configured to guide (e.g.,transmit, reflect, and/or diffract) the light reflected by the eye 115(referred to as reflected light in the following) into the opticalsensor 110. The reflected light may be incident onto the grating 120with various incidence angles, such as 0 degree (i.e., perpendicular tothe surface of the grating 120), 30 degrees, 45 degrees, 60 degrees, 70degrees, etc. A reflected light 131 that is substantially normallyincident onto the grating 120 may be diffracted by the gratingstructures 130 to a diffracted light 132. The optical sensor 110 may bepositioned to receive the diffracted light 132, and generate an image ofthe eye 115 based on the diffracted light 132. Such diffracted light 132may be referred to as a signal light or an eye-tracking signal. Areflected light 141 having an oblique incidence angle may not bediffracted by the grating structures 130 but may be reflected by asurface of the substrate 125 as a surface reflected light 142. In otherwords, there may be surface reflection at the surface of the substrate125. Because the optical sensor 110 is arranged to have a specificreceiving area, in addition to the signal light 132, the optical sensor110 may also receive the surface reflected light 142, which mayintroduce noise to the eye-tracking signal. Such surface reflected light142 received by the optical sensor 110 may be referred to as a noiselight or a noise. Thus, the optical sensor 110 may receive both thesignal light 132 and the noise light 142. The noise light 142 caused bythe surface reflection at the substrate 125 of the grating 120 mayadversely affect the quality of the image of the eye generated by theoptical sensor 110 based on the signal light 132. Therefore, theaccuracy of the eye-tracking may be degraded by the noise light 142.

FIG. 2 illustrates an eye-tracking system 200 to demonstrate an effectof surface reflection. The eye-tracking system 200 may be similar to theeye-tracking system 100, except that a grating 220 of the eye-trackingsystem 200 may include two substrates, a first substrate 205 and asecond substrate 215 for supporting and protecting purposes. It is notedthat in some embodiments, one of the two substrates may be omitted. Thatis, the eye-tracking system 200 may include only one substrate, as inthe eye-tracking system 100. Each of the first substrate 205 and thesecond substrate 215 may be similar to the substrate 125 shown in FIG.1.

The grating 220 may include a layer of grating structures 210 disposedbetween the first substrate 205 and the second substrate 215. Thegrating structures 210 may be any suitable grating structures. In someembodiments, the grating structures 210 may be in a form of a PVH layer(hence the grating structures 210 may also be referred to as a PVH layer210 for illustrative purposes), and the grating 220 may be a PVH gratingaccordingly. The PVH layer 210 may include a plurality of LC moleculesspatially orientated to realize an optical function of the PVH layer210. The PVH layer 210 may be configured to diffract a light via Braggdiffraction. The Bragg grating in the PVH layer 210 may be generated byadding a chiral dopant to the LCs to induce a helical twist along alight prorogation direction. The helix twist may be either left-handedor right-handed and, accordingly, the PVH layer 210 may be referred toas a left-handed or a right-handed PVH layer. In some embodiments, thePVH layer 210 may diffract circularly polarized light having a samehandedness as the helix twist of the PVH layer 210 and transmitcircularly polarized light having an orthogonal handedness. That is, aleft-handed PVH layer may diffract a left-handed circularly polarized(“LCP”) light and transmit a right-handed circularly polarized (“RCP”)light, while a right-handed PVH layer may diffract an RCP light andtransmit an LCP light.

The angle between the incident light and the diffracted light may dependon the wavelength of the incident light and the Bragg period of theBragg grating in the PVH layer 210. In some embodiments, depending onthe alignment of the LC molecules in the PVH layer, the PVH layer mayfurther converge or diverge the incident light. The PVH layer 210 may bealso referred to as, e.g., a “polarization sensitive grating,” a“polarization sensitive optical element,” a “liquid crystal grating,” ora “chiral liquid crystal element.” Although the grating 220 is shown asincluding flat surfaces, it is understood that in some embodiments, thegrating 220 may have a curved surface. For example, the substrates 205and 215 and the PVH layer 210 may each have a curved surface. Thedetailed structure of a PVH grating will be described in FIGS. 8A-8C.

For simplicity, the light source 105 and the eye 115 shown in FIG. 1(which may both be located on the same side as the optical sensor 110 inFIG. 2) are not shown in FIG. 2. In some embodiments, at least onewavelength in the spectrum of the light source 105 may correspond to theBragg period of the Bragg grating formed by the LC molecules in the PVHlayer 210. In some embodiments, the light emitted by the light source105 may have a wavelength in the IR spectrum and corresponding to theBragg period of the Bragg grating in the PVH layer 210. The wavelengthof the light may be, e.g., from about 800 nm to about 1600 nm, such asabout 850 nm, about 940 nm, or about 930 nm. The Bragg period of theBragg grating in the PVH layer 210 may be, e.g., from about 200 nm toabout 350 nm, or centered at about 288 nm or about 320 nm.

The light from the light source may be reflected by the eye 115 (shownin FIG. 1) and incident onto the grating 220 at different incidenceangles. The PVH layer 210 may be configured to diffract the reflectedlight incident onto the PVH layer 210 via Bragg diffraction when anincidence angle is within a first predetermined range, i.e., when aBragg condition is satisfied, and transmit the reflected light having anincidence angle outside of the first predetermined range as the Braggcondition is not satisfied. In some embodiments, the first predeterminedrange may be approximately from 0° to 60° in air, including 0° and 60°.In some embodiments, the substrate 215 may be a glass having arefractive index of 1.5, then the first predetermined range may beapproximately from 0° to 35° in the glass, including 0° and 35°. Forillustrative purposes, FIG. 2 shows a “Signal path” in which anunpolarized signal light 225 is substantially normally incident onto thegrating 220 (i.e., at a substantially 0° incidence angle). Because thesignal light 225 is incident substantially perpendicular to the surfaceof the substrates 205 and 215, the surface reflection (Fresnelreflectance) at the substrates 205 and 215 of the grating 220 may besubstantially zero.

On the other hand, the reflected light, which is incident onto the PVHlayer 210 at an incidence angle outside of the first predeterminedrange, may not be diffracted by the PVH layer 210 but may be reflectedat the surfaces of the substrates 205 and 215. The grating 220 may guidethe surface reflected light (i.e., noise light) towards the opticalsensor 110. However, not all the surface reflected light guided by thegrating 220 towards the optical sensor 110 may be received by theoptical sensor 110, because the optical sensor 110 has a specificreceiving area. In some embodiments, only the surface reflected lightthat is incident onto the grating 120 at an incidence angle within asecond predetermined range may be received by the optical sensor 110,while the surface reflected light that is incident onto the grating 210at an incidence angle outside of the second predetermined range may notbe received by the optical sensor 110. The surface reflected light thatis received by the optical sensor 110 may be referred to as a surfacereflection noise light. For illustrative purposes, FIG. 2 shows twonoise paths, “Noise path 1” and “Noise path 2.” In the noise paths,unpolarized lights 221 and 222 may be incident onto the grating 220 atan incidence angle within the second predetermined range, and may berespectively reflected at a surface of the first substrate 205 and asurface of the second substrate 215 towards the optical sensor 110 andreceived by the optical sensor 110.

When the first substrate 205 and the second substrate 215 are glasshaving refractive index of 1.5, the inventors found that the secondpredetermined range is about 45° to 75° in air. That is, the surfacereflection of the light that is incident onto the grating 220 within 45°to 75° of incidence angle may be received by the optical sensor 110,while the surface reflection of the light that is incident onto thegrating 210 outside of the 45° to 75° range of incidence angle may notbe received by the optical sensor 110. The inventors also found thatwhen the light is incident onto the grating 220 at an incidence anglewithin the second predetermined range of 45° to 75° in air, the surfacereflected light may be substantially s-polarized. FIG. 3 shows a plot ofFresnel transmittance (Ts and Tp) and reflectance (Rs and Rp) versus theincidence angle at an air-glass interface. Ts and Tp represent theFresnel transmittance for s-polarized light and p-polarized light,respectively. Rs and Rp represent the Fresnel reflectance for thes-polarized light and the p-polarized light, respectively. As shown inFIG. 3, due to the position and receiving area of the optical sensor110, the surface reflected light that can be received by the opticalsensor 110 was found to be the surface reflection of the light that hasan incidence angle in a range of 45° to 75° in air. In addition, withinsuch an incidence angle range, the surface reflected light is primarilys-polarized. As indicated by the curves for the Rs and Rp, within suchan incidence angle range, the reflectance for p-polarized light Rp issubstantially zero, whereas the reflectance for the s-polarized light isabout 0.1 to about 0.4.

Returning to FIG. 2, an effect of surface reflection in the grating 220is explained in the following. In the Noise path 1, the unpolarizedlight 221 may be incident onto the second substrate 215 at an incidenceangle in a range of 45° to 75° in air. The light 221 may propagatethrough the second substrate 215, the PVH layer 210 without beingdiffracted (because the PVH layer is designed to diffract light havingan incidence angle from 0° to 60° in air), and the first substrate 205,and then reflected by the top surface of the first substrate 205 tobecome an s-polarized light 230. The s-polarized light 230 may beincident onto the PVH layer 210. The PVH layer 210 may be a left-handedPVH layer that diffracts an LCP light and transmit an RCP light. Thus,an LCP portion of the s-polarized light 230 may be diffracted by the PVHlayer 210 to become an LCP light 231 propagating towards the firstsubstrate 205, and an RCP light portion of the s-polarized light may betransmit through the PVH layer 210 to be an RCP light 233 propagatingtowards the second substrate 215. The RCP light 233 may be refracted atthe bottom surface of the second substrate 215 (the surface facing theeye 115 or the optical sensor 110) to become an RCP light 245, which isreceived by the optical sensor 110. In the Noise path 2, the unpolarizedlight 222 may be incident onto the second substrate 215 at an incidenceangle in a range of 45° to 75° in air. The unpolarized light 222 may bereflected at the bottom surface of the second substrate 215 to become ans-polarized light 255.

In the Signal path, the unpolarized signal light 225 may besubstantially normally incident onto the second substrate 215, anddiffracted by the PVH layer 210 to become an LCP light 240. Forsimplicity of illustration and discussion, the RCP portion of theunpolarized signal light 225 that transmits through the PVH layer 210towards the first substrate 205 is omitted in FIG. 2. The LCP light 240may be refracted at the bottom surface of the second substrate 215 tobecome an LCP light 250. The RCP light 245 from the Noise path 1, thes-polarized light 255 from the Noise path 2, and the LCP light 250 fromthe signal path may all be received by the optical sensor 110, based onwhich an image of the eye 115 may be generated. The s-polarized light255 and the RCP light 245 may be the surface reflection noise light,which may reduce the signal-to-noise ratio of the eye-tracking signal250. As a result, the accuracy of the eye-tracking may be degraded.

FIG. 4 illustrates an eye-tracking system 400 with a surface reflectionnoise reduction mechanism, according to an embodiment of the presentdisclosure. The eye-tracking system 400 includes elements similar tothose included in the eye-tracking systems 100 and 200 shown in FIG. 1and FIG. 2. The descriptions of the similar or the same elements areomitted. As shown in FIG. 4, compared to the eye-tracking system 200shown in FIG. 2, the eye-tracking system 400 may further include awaveplate 410 and a polarizer 460 in addition to a grating 420. The PVHlayer 210 of the grating 420 has a first side facing the eye 115 (or theoptical sensor 110) and an opposing second side, and the waveplate 410may be disposed at the first side of the PVH layer 210. For illustrativepurposes, FIG. 4 shows the waveplate 410 disposed between the PVH layer210 and the second substrate 215. That is, the second substrate 215 hasa first side facing the eye 115 (or the optical sensor 110) and anopposing second side, and the waveplate 410 may be disposed at thesecond side of the second substrate 215. In some embodiments, thewaveplate 410 may be disposed at the first side of the second substrate215 facing the optical sensor 110.

In some embodiments, the waveplate 410 may be a quarter-wave plate 410for the infrared spectrum. The quarter-wave plate 410 may be configuredto convert a circularly polarized light to a linearly polarized lightand vice versa for an infrared spectrum. In some embodiments, for anachromatic design, the quarter-wave plate 410 may include a plurality oflayers of one or more birefringent materials (e.g., polymer or LCmaterials) to produce quarter wave birefringence across a wide spectrarange. In some embodiments, for a monochrome design, an angle between apolarization axis (i.e., fast axis) of the quarter-wave plate 410 and apolarization direction of the incident linearly polarized light may beapproximately 45 degrees. As a person having ordinary skills in the artcan appreciate, the quarter-wave plate 410 for the infrared spectrum maybe a half-wave plate for the visible spectrum. For example, if theinfrared wavelength is 900 nm, the quarter-wave plate 410 for 900 nmwavelength in infrared range may be approximately a half-wave plate for450 nm wavelength in the visible range.

As shown in FIG. 4, in the Noise path 1, an unpolarized light 421 may beincident onto the second substrate 215 at an incidence angle in a rangeof 45° to 75° in air. After being transmitted through the secondsubstrate 215, the quarter-wave plate 410, the PVH layer 210 (withoutbeing diffracted), and the first substrate 205, the unpolarized noiselight 421 may be reflected at a top surface of the first substrate 205to be an s-polarized light 430, which propagates towards the PVH layer210. The PVH layer 210 may diffract an LCP portion of the s-polarizedlight 430 to be an LCP light 435, and transmit an RCP portion of thes-polarized light 430 to be an RCP light 440 towards the quarter-waveplate 410. The quarter-wave plate 410 may convert the RCP light 440 toan s-polarized light 445. The s-polarized light 445 may be refracted atthe bottom surface of the second substrate 215 to become an s-polarizedlight 455.

The polarizer 460 may be disposed in front of the optical sensor 110 inoptical series, i.e., upstream of the optical sensor 110 in the opticalpath. In some embodiments, the polarizer 460 may be disposed between thegrating 420 and the optical sensor 110. Optical series refers torelative positioning of a plurality of optical elements, such thatlight, for each optical element of the plurality of optical elements, istransmitted by that optical element before being transmitted by anotheroptical element of the plurality of optical elements. Moreover, orderingof the optical elements does not matter. For example, optical element Aplaced before optical element B, or optical element B placed beforeoptical element A, are both in optical series. Similar to electriccircuitry design, optical series represents optical elements with theiroptical properties compounded when placed in series.

The polarizer 460 may be configured to block a light having a firstpolarization and transmit a light having a second polarization differentfrom the first polarization. In some embodiments, the secondpolarization may be orthogonal to the first polarization. For example,the polarizer 460 may be configured to block an s-polarized light andtransmit a p-polarized light. Therefore, the s-polarized light 455 maybe blocked by the polarizer 460, i.e., the optical sensor 110 may notreceive the s-polarized light 455.

In the Noise path 2, an unpolarized light 422 may be incident onto thesecond substrate 215 at an incidence angle in a range of 45° to 75° inair. The unpolarized light 422 may be reflected by the bottom surface ofthe second substrate 215 to become an s-polarized light 450, which isalso blocked by the polarizer 460. Accordingly, the optical sensor 110may not receive the s-polarized light 450.

In the Signal path, an unpolarized signal light 425 may be substantiallynormally incident onto the grating 420 (i.e., the incidence angle isabout 0 degree). The signal light 425 may propagate through the secondsubstrate 215 and the quarter-wave plate 410, and substantially normallyincident onto the PVH layer 210. An LCP portion of the signal light 425may be diffracted by the PVH layer 210 to become an LCP light 465propagating towards the quarter-wave plate 410, and an RCP portion ofthe signal light 425 may be transmit through the PVH layer 210propagating towards the first substrate 205, which is not shown in FIG.4 for simplicity. The quarter-wave plate 410 may convert the LCP light465 to a p-polarized light 470. The p-polarized light 470 may berefracted at the bottom surface of the second substrate 215 to become ap-polarized light 475. Because the polarizer 460 transmits a p-polarizedlight and blocks an s-polarized light, the p-polarized light 475 may betransmitted through the polarizer 460 and received by the optical sensor110.

Thus, the optical sensor 110 may be configured to receive only thep-polarized light 475 coming from the Signal path, and not receive thes-polarized light 455 and the s-polarized light 450 coming from theNoise path 1 and Noise path 2, respectively. As a result, the noise inthe eye-tracking signal may be reduced or eliminated, thereby improvingthe accuracy of eye-tracking. For example, when the optical sensor 110includes a camera, the ghost image in the image of the eye 115 capturedby the camera may be reduced or eliminated. In some embodiments, thegrating 420 may further include visible antireflection to gain moresignal in the IR Fresnel reflection.

FIG. 5A illustrates an experimental result of the conventionaleye-tracking system 200 in FIG. 2 that does not include a mechanism toreduce the surface reflection noise. FIG. 5B illustrates an experimentalresult of the eye-tracking system 400 in FIG. 4 that includes amechanism to reduce the surface reflection noise. In the experiment, forillustrative purposes, the eye 115 is replaced by a piece of paper withtexts. FIG. 5A shows an image captured by the optical sensor 110, whichis a camera. As shown in FIG. 5A, noise (ghost image) caused by surfacereflection of the grating 210 exists in the area framed by a dashedrectangle 505. As a compassion, as shown in FIG. 5B, the surfacereflection noise light (ghost image) is significantly reduced in adashed rectangle 510. With the reduced surface reflection noise, theaccuracy of the eye-tracking may be enhanced.

It should be noted that in FIGS. 2-4, the first predetermined range ofthe incidence angle at the grating 220 and the grating 420 is about 0°to 60° in air, and the second predetermined range of the incidence angleat the grating 220 and the grating 420 is about 45° to 75° in air, whichis for illustrative purposes and is not intended to limit the scope ofthe present disclosure. The first predetermined range of the incidenceangle may be determined according to various factors, such as thewavelength of the incident light, the grating period, and/or the slantangle of the grating structures, etc. The second predetermined range ofthe incidence angle may be determined according to various factors, suchas the refractive indices of the materials of the grating structures andthe substrate, the wavelength of the incident light, the grating period,the slant angle of the grating structures, the position of the opticalsensor, and/or the receiving area of the optical sensor, etc. Thesurface noise reduction mechanism provided by the present disclosure isconfigured such that the surface reflected noise light and the signallight incident onto the polarizer have different polarizations, suchthat the polarizer 460 disposed in front of the optical sensor 110 inoptical series may block the surface reflected noise light having onepolarization and transmit the signal light having another differentpolarization, thereby improving the signal-to-noise ratio of theeye-tracking. In some embodiments, configuring the surface noisereduction mechanism such that the surface reflected noise light and thesignal light are received by the optical sensor 110 with differentpolarizations may be include configuring a position and a receiving areaof the optical sensor and configuring the parameters (such as thewavelength of the incident light, the grating period, and/or the slantangle of the grating structures) of the grating 420, such that thesurface reflection noise light to be incident onto the polarizer 460disposed in front of the optical sensor 110 in optical series may besubstantially s-polarized.

FIG. 4 shows that the eye-tracking system 400 includes a polarizationselective/sensitive grating, e.g., a PVH grating, configured to guidethe light reflected by the eye 115 towards the optical sensor 110. Insome embodiments, the eye-tracking system 400 may include anon-polarization selective/sensitive grating configured to guide thelight reflected by the eye 115 towards the optical sensor 110. Anexemplary structure is shown in FIG. 6.

FIG. 6 illustrates another eye-tracking system 600 that includes asurface reflection noise reduction mechanism. The similarities betweenFIG. 6 and FIG. 4 are not repeated here, while certain difference isexplained. As shown in FIG. 6, the eye-tracking system 600 may include agrating 620, a polarizer 660, and the optical sensor 110. The grating620 may guide a light, which is a light emitted from a light source toilluminate an eye 115 and reflected by the eye 115, towards the opticalsensor 110. The polarizer 660 may be arranged in front of the opticalsensor 110 in optical series. The polarizer 660 may be configured toblock an s-polarized light and transmit a p-polarized light. Forexample, the polarizer 660 may be configured to block a light reflectedby a surface of a substrate of the grating 620, which may besubstantially s-polarized, and transmit a p-polarized portion of a lightdiffracted by the grating 620. The optical sensor 110 may be configuredto receive the p-polarized portion of the light transmitted through thepolarizer 660.

The grating 620 may include a first substrate 605, a second substrate615, and a holographic optical element (“HOE”) 610 disposed between thefirst substrate 605 and the second substrate 615. In some embodiments,one of the first substrate 605 and the second substrate 615 may beomitted. The substrate 605 and 615 may be similar to the substrates 205and 215. The HOE 610 may be a non-polarization selective/sensitiveelement, i.e., a polarization insensitive element. In some embodiments,the HOE 610 may be a volume Bragg grating (“VBG”) element (hence the HOE610 is also referred to as VBG 610, and the grating 620 is also referredto as VBG grating 620).

The VBG 610 may be configured such that only light having an incidenceangle within a third predetermined range may be diffracted via Braggdiffraction, and light having an incidence angle outside of the firstpredetermined range may not be diffracted (e.g., may be transmittedthrough without diffraction). In some embodiments, the firstpredetermined range of the incidence angle may be approximately from 0°to 60° in air, including 0° and 60°. In some embodiments, the substrate615 may be a glass having a refractive index of 1.5, then the firstpredetermined range may be approximately from 0° to 35° in the glass,including 0° and 35°.

As shown in FIG. 6, in the Noise path 1, an unpolarized light 621 may beincident onto the second substrate 615 at an incidence angle in a rangeof 45° to 75° in air. The unpolarized light 621 may propagate throughthe second substrate 615, the VBG 610 (without being diffracted), andmay be reflected by a top surface of the first substrate 605. Asdiscussed above in connection with FIG. 3, the reflected light 630 maybe an s-polarized light. The s-polarized light 630 may propagate throughthe first substrate 205, the VBG 610 (without being diffracted), and thesecond substrate 615, and become an s-polarized light 650. In the Noisepath 2, an unpolarized noise light 622 may be incident onto the secondsubstrate 615 at an incidence angle in a range of 45° to 75° in air. Theunpolarized noise light 622 may be reflected by a bottom surface of thesecond substrate 615 and become an s-polarized light 655. In the Signalpath, an unpolarized signal light 625 may be substantially normallyincident onto the grating 620. The signal light 625 may prorogatethrough the second substrate 215 and be diffracted by the VBG 610 tobecome an unpolarized signal light 665. The unpolarized signal light 665may propagate through the second substrate 615 and become an unpolarizedsignal light 675.

The polarizer 660 may be configured to block an s-polarized light andpass a p-polarized light. Thus, the s-polarized light 675 and thes-polarized portion of the unpolarized signal light 675 may be blocked,while the p-polarized portion of the unpolarized signal light 675 may betransmitted through the polarizer 660 to be received by the opticalsensor 110. When the optical sensor 110 is a camera, the optical sensor110 may generate an image of the eye 115 based on the p-polarizedportion of the unpolarized light 675 for eye-tracking purpose. Thus, thenoise caused by the surface reflection at the surface of the substratesof the grating 620 may be suppressed, and the signal-to-noise ratio ofthe eye-tracking signal may be improved. Accordingly, the accuracy ofthe eye-tracking may be improved.

FIG. 7A illustrates an image generated by a conventional eye-trackingsystem without a surface reflection noise reduction mechanism. Forexample, the conventional eye-tracking system may be similar to theeye-tracking system 600, except that the conventional eye-trackingsystem may not include the polarizer 660. As shown in a dashed rectangle705, a ghost image caused by the surface reflection can be observed.FIG. 7B illustrate an image generated by the eye-tracking system 600that includes the polarizer 660. As shown in FIG. 7B, surface reflectionnoise (ghost image) is significantly suppressed in the image areaindicated by a dashed rectangle 710.

FIGS. 8A-8B schematically show an example PVH layer 800 that may beincluded in the grating 220 and grating 420, as shown in FIG. 2 and FIG.4, and the grating 120 shown in FIG. 1. FIG. 8A is a cross-sectionalview of the PVH layer 800 in the x-z plane. FIG. 8B is a partial planview 850 of the PVH layer 800 in the x-y plane along the x-axis from acenter region of the PVH layer 800 to an edge region of the PVH layer800. The optical function of a PVH layer may be determined based on themanipulation of optic axes of the liquid crystal (“LC”) molecules in thePVH layer. Hereinafter, an orientation of the optic axis of an LCmolecule is also referred to as an orientation or alignment of the LCmolecule. The manipulation of optic axes of the LC molecules in the PVHlayer is a three-dimensional (“3D”) alignment of the LC molecules. A PVHlayer consistent with the present disclosure can deflect a light viaBragg diffraction. The Bragg grating in the PVH layer may be created byadding a chiral dopant to induce a helical twist along a certaindirection, e.g., an h-axis direction shown in FIG. 8A.

As shown in FIG. 8A, the LC (or more broadly speaking, birefringentmaterial because a reactive mesogen may also be employed) exhibits ahelical structure with a period length of Λy (or one half of the pitchlength p) along y-axis. The LC molecules may exhibit uniform molecularrotation with respect to a slanted helical axis, e.g., an h-axisdirection. The period Λy (or pitch length p=2Λ_(y)) may be adjusted bycontrolling the helical twist power (“HTP”) and concentration of thechiral dopant. Further, as shown in FIG. 8B, an in-plane periodicity inthe x-y plane is also introduced into the PVH layer 800 by, e.g.,modifying the surface alignment of the PVH layer 800 to provide arotation of the LC molecules in the x-y plane. The optic axis of the LCmolecules may be changed in a linearly repetitive pattern from a center802 to an edge 804 of the PVH layer 800, with a uniform pitch Ax alongthe x-axis. Such a scheme generates a series of slanted and periodicalrefractive index planes with a slanted angle φ=±arctan(Λ_(y)/Λ_(x)), asshown in FIG. 8A. The distance between neighboring slanted lines is theBragg period Λ_(B) of the Bragg grating formed by the LC molecules inthe PVH layer 800. The Bragg period Λ_(B) may depend on the z-axisperiod Λ_(z) of the LC molecules and the slanted angle φ of the Braggplanes with respect to a surface of the PVH layer 800.

It should be noted that, the partial plan view of the PVH layer 800 inthe x-y plane in FIG. 8B is for illustrative purposes and is notintended to limit the scope of the present disclosure. In someembodiments, the LC molecules in the x-y plane may be configured withother orientation to realize a different optical function. FIG. 8C isanother partial plan view 860 of the PVH layer shown in FIG. 8A in thex-y plane, according to an embodiment of the disclosure. As shown inFIG. 8C, the PVH layer 800 may create a respective lens profile via thein-plane (x-y plane) orientation (azimuth angle θ) of the LC molecules,in which the phase difference T=2θ. In the PVH layer 800, the azimuthangles of the LC molecules may change continuously from a center 812 toan edge 814 of the PVH layer 800, with a varied period Λ, i.e., adistance between two LC molecules whose azimuth angles differ from eachother by 180°. The lens of the PVH layer 800 may include a certainsymmetry in the arrangement of the LC molecules about an optical axis ofthe PVH layer 800, which, for example, may pass through the center 812of the PVH layer 800. Depending on the alignment of the LC molecules inthe PVH layer 800, the PVH layer 800 may further converge or diverge theincident light in addition to diffracting the incident light.

FIG. 9A illustrates a schematic diagram of an HMD 900 according to anembodiment of the disclosure. In some embodiments, the HMD 900 may bereferred to as a near-eye display (NED). The HMD 900 may present mediato a user. Examples of media presented by the HMD 900 include one ormore images, video, audio, or some combination thereof. In someembodiments, audio is presented via an external device (e.g., speakersand/or headphones) that receives audio information from the HMD 900, aconsole (not shown), or both, and presents audio data based on the audioinformation. The HMD 900 acts as a virtual reality (VR) device, anaugmented reality (AR) device or a mixed reality (MR) device, or somecombination thereof. In some embodiments, when the HMD 900 acts as anaugmented reality (AR) or a mixed reality (MR) device, portions of theHMD 900 and its internal components may be at least partiallytransparent.

As shown in FIG. 9A, the HMD 900 may include a frame 905, a display 910,and an eye-tracking system 930 (not drawn in FIG. 9A). Certain device(s)may be omitted, and other devices or components may also be included.The frame 910 may include any appropriate type of mounting structure toensure the display assembly 920 to be viewed as a near-eye display (NED)by a user. The frame 905 may be coupled to one or more optical elementswhich together display media to users. In some embodiments, the frame905 may represent a frame of eye-wear glasses. The display 910 isconfigured for users to see the content presented by the HMD 900. Asdiscussed below in conjunction with FIG. 9B, the display 910 may includeat least one display assembly (not shown) for directing image light toan eye of the user.

FIG. 9B is a cross-section 950 of the HMD 900 shown in FIG. 9A accordingto an embodiment of the disclosure. The display 910 may include at leastone waveguide display assembly 915. An exit pupil 925 may be a locationwhere the eye 920 is positioned in an eye-box region when the user wearsthe HMD 900. For purposes of illustration, FIG. 9B shows the crosssection 950 associated with a single eye 920 and a single waveguidedisplay assembly 915, but in alternative embodiments not shown, anotherdisplay assembly which is separate from the waveguide display assembly915 shown in FIG. 9B, may provide image light to an eye-box located atan exit pupil of another eye of the user.

The waveguide display assembly 915, as illustrated below in FIG. 9B, isconfigured to direct the image light to an eye-box located at the exitpupil 925 of the eye 920. The waveguide display assembly 915 may becomposed of one or more materials (e.g., plastic, glass, etc.) with oneor more refractive indices that effectively minimize the weight andwiden a field of view (FOV) of the HMD 900. In some embodiments, thewaveguide display assembly 915 may be a component (e.g., the display910) of the HMD 900. In some embodiments, the waveguide display assembly915 may be part of some other NED, or other system that directs displayimage light to a particular location. As shown in FIG. 9B, the waveguidedisplay assembly 915 may be for one eye 920 of the user. The waveguidedisplay assembly 915 for one eye may be separated or partially separatedfrom the waveguide display assembly 915 for the other eye. In certainembodiments, a single waveguide display assembly 915 may be used forboth eyes 920 of the user.

In some embodiments, the HMD 900 may include one or more opticalelements between the waveguide display assembly 915 and the eye 920. Theoptical elements may act to, e.g., correct aberrations in image lightemitted from the waveguide display assembly 915, magnify image lightemitted from the waveguide display assembly 915, some other opticaladjustment of image light emitted from the waveguide display assembly915, or some combination thereof. The example for optical elements mayinclude an aperture, a Fresnel lens, a convex lens, a concave lens, afilter, or any other suitable optical element that affects image light.In some embodiments, the HMD 900 may include an adaptive dimming device930, which includes a global or local dimming element. In someembodiments, the dimming element may be electrically or opticallytunable. The dimming element may dynamically adjust the transmittance ofthe see-through view observed through the HMD 900, thereby switching theHMD 900 between a VR device and an AR device or between a VR device anda MR device. In some embodiments, along with switching between the AR/MRdevice and the VR device, the dimming element may be used in the ARdevice to mitigate difference in brightness of the see-through view andthe virtual image. In some embodiments, the dimming element maydynamically attenuate a light from the real-world environment dependingon brightness of the real-world environment, thereby adjusting thebrightness of the see-through view.

The eye-tracking system 930 may be any one of the disclosed eye-trackingsystems, such as the eye-tracking system 400 or 600 shown in FIG. 4 andFIG. 6, which may include a surface reflection noise reductionmechanism, as described above. With the eye-tracking system 400 or 600,more accurate eye-tracking may be provided by the HMD 900. Informationobtained in the accurate eye tracking may be used for determining thetype of information to be presented to the user of the HMD 900 and/orthe arrangement of the displayed content on a display screen of the HMD900, addressing vergence/accommodation conflict, and improving displayquality and performance of the HMD 900. Accordingly, user experience ofthe HMD 900 may be enhanced.

The foregoing description of the embodiments of the disclosure have beenpresented for the purpose of illustration. It is not intended to beexhaustive or to limit the disclosure to the precise forms disclosed.Persons skilled in the relevant art can appreciate that manymodifications and variations are possible in light of the abovedisclosure.

Some portions of this description describe the embodiments of thedisclosure in terms of algorithms and symbolic representations ofoperations on information. These algorithmic descriptions andrepresentations are commonly used by those skilled in the dataprocessing arts to convey the substance of their work effectively toothers skilled in the art. These operations, while describedfunctionally, computationally, or logically, are understood to beimplemented by computer programs or equivalent electrical circuits,microcode, or the like. Furthermore, it has also proven convenient attimes, to refer to these arrangements of operations as modules, withoutloss of generality. The described operations and their associatedmodules may be embodied in software, firmware, hardware, or anycombinations thereof.

Any of the steps, operations, or processes described herein may beperformed or implemented with one or more hardware or software modules,alone or in combination with other devices. In one embodiment, asoftware module is implemented with a computer program product includinga non-transitory computer-readable medium containing computer programcode, which can be executed by a computer processor for performing anyor all of the steps, operations, or processes described.

Embodiments of the disclosure may also relate to an apparatus forperforming the operations herein. This apparatus may be speciallyconstructed for the required purposes, and/or it may comprise ageneral-purpose computing device selectively activated or reconfiguredby a computer program stored in the computer. Such a computer programmay be stored in a non-transitory, tangible computer readable storagemedium, or any type of media suitable for storing electronicinstructions, which may be coupled to a computer system bus.Furthermore, any computing systems referred to in the specification mayinclude a single processor or may be architectures employing multipleprocessor designs for increased computing capability.

Embodiments of the disclosure may also relate to a product that isproduced by a computing process described herein. Such a product mayinclude information resulting from a computing process, where theinformation is stored on a non-transitory, tangible computer readablestorage medium and may include any embodiment of a computer programproduct or other data combination described herein. Finally, thelanguage used in the specification has been principally selected forreadability and instructional purposes, and it may not have beenselected to delineate or circumscribe the inventive subject matter. Itis therefore intended that the scope of the disclosure be limited not bythis detailed description, but rather by any claims that issue on anapplication based hereon. Accordingly, the disclosure of the embodimentsis intended to be illustrative, but not limiting, of the scope of thedisclosure, which is set forth in the following claims.

1. An optical A system, comprising: a polarizer; a grating structureconfigured to diffract a first input light as a first output lighttoward the polarizer; and at least one substrate coupled to the gratingstructure, a surface of the at least one substrate configured to reflecta second input light as a substantially s-polarized light toward thepolarizer, wherein the first input light and the second input light areincident onto the grating structure and the at least one substrate,respectively, from a side where the polarizer is located, and whereinthe polarizer is configured to transmit the first output lightdiffracted by the grating structure and block the substantiallys-polarized light reflected by the surface of the at least onesubstrate.
 2. The system of claim 1, further comprising: an opticalsensor disposed facing the grating structure and configured to receivethe first output light that is diffracted by the grating structure andtransmitted through the polarizer.
 3. The system of claim 1, wherein thesubstantially s-polarized light reflected by the surface of the at leastone substrate and the first output light diffracted by the gratingstructure have different polarizations.
 4. (canceled)
 5. The system ofclaim 1, wherein the grating structure includes a polarization volumehologram (“PVH”) layer.
 6. The system of claim 5, wherein the gratingstructure including the PVH layer is configured to diffract a polarizedlight having a first predetermined handedness, and to transmit apolarized light having a second predetermined handedness that isopposite to the first predetermined handedness.
 7. The system of claim6, further comprising a waveplate disposed between the grating structureand the polarizer.
 8. The system of claim 7, wherein the waveplate is aquarter-wave plate for an infrared spectrum.
 9. (canceled)
 10. Anoptical system, comprising: at least one substrate; a grating structurecoupled to the at least one substrate, the grating structure configuredto diffract a first light having an incidence angle within apredetermined range; and a polarizer configured to transmit the firstlight diffracted by the grating structure and block a second lightreflected by a surface of the at least one substrate, wherein thegrating structure is a polarization volume hologram (“PVH”) grating,wherein the at least one substrate includes a first substrate and asecond substrate arranged opposite to the first substrate, wherein thefirst substrate and the second substrate are disposed at opposing firstand second sides of the PVH grating to sandwich the PVH grating, and aquarter-wave plate disposed between the PVH grating and the firstsubstrate.
 11. The system of claim 1, wherein the grating structure is avolume Bragg grating (“VBG”).
 12. The system of claim 1, wherein thefirst input light has an incidence angle within a first predeterminedrange of 0 degree to 35 degrees.
 13. The system of claim 1, furthercomprising a light source configured to emit an infrared light, whereinthe first input light and the second input light are both infraredlight.
 14. An eye-tracking system, comprising: a light source configuredto emit a light to illuminate an eye of a user; a polarizer; a gratingstructure disposed facing the eye and configured to diffract a firstinput light reflected from the eye as a first output light toward thepolarizer; and at least one substrate coupled to the grating structure,a surface of the at least one substrate configured to reflect a secondinput light reflected from the eye as a substantially s-polarized lighttoward the polarizer, wherein the first input light and the second inputlight are incident onto the grating structure and the at least onesubstrate, respectively, from a side where the polarizer is located, andwherein the polarizer is configured to transmit the first output lightdiffracted by the grating structure, and to block the substantiallys-polarized light reflected by the surface of the at least onesubstrate; and an optical sensor configured to receive the first outputlight transmitted through the polarizer and generate an image of the eyebased on the received first output light.
 15. The eye-tracking system ofclaim 14, wherein the first output light diffracted by the gratingstructure and the substantially s-polarized light reflected by thesurface of the at least one substrate have different polarizations. 16.(canceled)
 17. The eye-tracking system of claim 14, wherein the gratingstructure includes a polarization volume hologram (“PVH”) layerconfigured to diffract a polarized light having a first predeterminedhandedness, and to transmit a polarized light having a secondpredetermined handedness that is opposite to the first predeterminedhandedness.
 18. The eye-tracking system of claim 17, further comprisinga quarter-wave plate for an infrared spectrum disposed between thegrating structure and the polarizer.
 19. (canceled)
 20. The eye-trackingsystem of claim 14, wherein the grating structure is a volume Bragggrating (“VBG”).
 21. The system of claim 1, wherein the second inputlight has an incidence angle within a second predetermined range of 45°to 75° in air.
 22. The system of claim 1, wherein the first input lightis incident onto the grating structure from air, and the second inputlight is incident onto the at least one substrate from air.
 23. Theoptical system of claim 10, wherein the first light is incident onto thegrating structure from air, and the second light is incident onto thesurface of the at least one substrate from air at an incidence angle ina range of 45° to 75° in air.
 24. (canceled)